This invention relates generally to limiters for use in fusion devices; and, more particularly to fusion devices employing RF (radiofrequency) heating.
Limiters are used in fusion devices generally to position the plasma away from the vacuum vessel walls, to remove useful heat, and to remove both hydrogen and impurities (pumped limiters). The heat loads in present day tokamaks place stringent requirements on their limiters. Limiters can be expected to receive power loadings in excess of 10.sup.4 W/cm.sup.2 and a total deposited energy greater than 10.sup.5 Joules. Recently, two plasma materials interactions, which are peculiar to RF heated plasmas, have been discovered to cause detrimental effects to limiters. First, during ICRF heating in the H.sup.+ second harmonic and H.sup.+ minority regimes, fast ion loss to the plasma edge has been reported (D. M. Manos et al, J. Nucl. Mater. 111 & 112 (1982) 30). Small carbon probes placed in the plasma edge have experienced enhanced erosion attributed to synergistic interaction involving both the bulk plasma and energetic ions. Second, during LHCD experiments on PLT (Princeton Large Torus) by the inventors, small (.about.1 cm.sup.2) incandescent hot spots have been observed on the limiters. These hot spots have been attributed to energetic electron impact and result in a carbon influx into the plasma which quenches the current drive.
The higher the V.sub..parallel. (parallel velocity) of a species in the plasma, the shorter its scrape-off distance. For electrons of 100 keV parallel energy, the scrape-off distance is about 0.1 cm. This results in a concentrated heat load by the lost energetic electrons on the part of the limiter nearest the plasma edge.
Current limiter designs include the so-called "mushroom" shaped limiter and the rotating disk limiter. The mushroom-shaped limiters have either a concave or convex front face (the portion of the limiter closest to the plasma). The disk-shaped limiter positions the disk edge closest to the plasma. Both types of limiter employ rotation (the mushroom limiter rotating about its "stem" and the disk about its center) to distribute heat loads. The major disadvantages of the convex mushroom limiter are: inadequate ballistic reflection of the incident plasma and inability to handle power loads due to relativistic electrons. The major disadvantages of the concave mushroom limiter are: inadequate heat load distribution to the main limiter mass and inadequate surface area accepting the heat flux. The major disadvantages of the disk limiter are: lack of compactness and inadequate reflection of the incident plasma.
Therefore it is an object of the present invention to provide a limiter than can handle high power fluxes and prevent the evolution of hot spots.
It is another object of the present invention to provide a limiter suitable for use in a RF heating environment and that can effectuate hydrogen removal and impurity control.
It is yet another object of the present invention to provide a limiter capable of bearing the high heat load due to near relativistic electrons and suprathermal ions.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.